Photodetectors capable of detecting a single-photon (a single “particle” of optical energy) are useful in many applications. To date, most of these applications have relied on the use of single-photon detectors such as photomultiplier tubes (PMTs) or single-photon avalanche detectors (SPADs) that are silicon-based, and are therefore capable of efficiently detecting only photons that have a wavelength within the range of approximately 250 nanometers (nm) to approximately 900 nm. New applications are emerging, however, that require single-photon detectors that can operate at high speed (>1 MHz) and at longer wavelengths (>1000 nm). Such devices would find use in areas such as: quantum information processing, quantum computing, quantum cryptography, and quantum teleportation and communications; low-light-level imaging and other high-performance imaging applications; and others. Unfortunately, currently available SPADs do not have the combination of high operational speed and wavelength range required for many of these applications.
An avalanche photodiode is one type of photodetector that is capable of providing extremely high sensitivity. In fact, an avalanche photodiode can be made sensitive enough to provide a discernable output signal upon the receipt even a single photon.
Avalanche photodiodes derive their name from the manner in which their output signal is created. When photons are absorbed by an avalanche photodiode, their energy frees bound charge carriers (electrons and holes) that then become free-carrier pairs. In the presence of an electric field (due to a bias voltage applied to the photodiode), these free-carriers are accelerated through a region of the avalanche photodiode referred to as the “multiplication region.” As the free carriers travel through the multiplication region, they collide with other carriers bound in the atomic lattice of the semiconductor, thereby generating more free carriers through a process called “impact ionization.” These new free-carriers also become accelerated by the applied electric field and generate yet more free-carriers. This avalanche event can occur very rapidly and efficiently and it is possible to generate several hundred million free-carriers from a single absorbed photon in less than one nanosecond.
An avalanche photodiode is characterized by its “breakdown voltage.” A photodiode's breakdown voltage is a bias level above which free-carrier generation can become self-sustaining and result in run-away avalanche. To enable it for single-photon detection, an avalanche photodiode is biased with a voltage that is larger than its breakdown voltage. This is referred to as “arming” the photodiode. Once the photodiode is armed, a single free carrier pair created by the absorption of a single photon can create a runaway avalanche resulting in an easily detectable macroscopic current.
Once a signal has been generated as a result of the absorption of a photon, it is necessary to stop the self-sustained avalanche so that the photodiode can be rearmed to detect another photon. To halt the avalanche process, the bias voltage of the avalanche photodiode is reduced below its breakdown voltage. This process is referred to as “quenching” the avalanche photodiode.
The rate at which a single-photon detector can be operated is determined by two factors: 1) how fast an avalanche event can be stopped once it is detected; and 2) how fast the avalanche detector can be re-armed once the avalanche event has been stopped.
Although quenching stops the avalanche process, not all free carriers are instantaneously swept out of the avalanche region. Instead, some carriers become trapped in trap energy states in the multiplication region that exist due to crystalline defects or other causes. These trapped carriers become released in a temporally random manner as a function of temperature, the type of trap state, and the applied bias voltage. If a trapped carrier is released after the SPAD has already been re-armed (by biasing above the breakdown voltage), it is likely to initiate impact ionization in the same manner as the free-carriers that resulted from the absorption of a photon. As a result, the detrapping of a carrier is likely to result in the detection of an electrical signal that is not due to the absorption of a photon. A “false” electrical signal that occurs in the absence of photon absorption is referred to as a “dark count.” Dark counts due to the detrapping of trapped carriers are referred to as “afterpulses.”
Because dark counts constitute noise in a single-photon avalanche detector, they degrade its sensitivity. Trapped charges will generally become free carriers in random fashion due to, for example, thermal emission from the trap. One approach for achieving high sensitivity detection is to simply delay rearming after quenching to allow trapped charges a sufficient period of time to detrap while the SPAD remains unarmed. In this way, the resulting afterpulse rate can be reduced to an acceptable limit. This approach, however, results in an undesirably long period of time when the single-photon detector is inoperable. To date, afterpulsing has limited the use of single-photon detectors that are sensitive to wavelengths of light greater than 1000 nm to maximum rates of approximately 1 MHz.
Several approaches for reducing afterpulse effects could be considered in order to increase the operation rate of single-photon detectors. These include 1) actively inducing rapid detrapping of trapped charges; 2) limiting the number of free-carriers that flow through the multiplication region during an avalanche event; and 3) stifling the detrapping of trapped charges.
Trapped charges can be induced to more rapidly detrap by elevating the temperature of the photodiode or energizing the carriers by illuminating them with light at a different wavelength. These approaches, however, have shown very limited success: elevating the photodiode temperature imposes a severe tradeoff by increasing the dark count rate, and sub-bandgap illumination has not yet been shown to effectively induce carrier detrapping. In addition, their added cost and complexity make these approaches undesirable in many applications.
It is theoretically possible to limit the number of free-carriers that flow through the multiplication region during an avalanche event by actively quenching the photodiode using external circuitry. Practical limitations of even state-of-the-art electronics (e.g., circuit gain-bandwidth limitations) make this approach infeasible in many applications, however. In addition, the added capacitance associated with the external circuitry can slow the rate at which an avalanche photodiode can recharge while being rearmed. The rate at which a SPAD can be rearmed is dictated by the product of the photodetector's capacitance and resistance (referred to as the RC time constant). For high speed operation (i.e., >1 MHz), this RC time constant must be less than about 1 microsecond. As a result, any capacitance associated with external electronics that adds to the capacitance of the SPAD itself is generally undesirable.
It has been demonstrated that the number of free-carriers can be limited successfully through passive quenching in a device that includes an integrated resistor as part of the photodiode structure itself. This approach is described in papers that include “MRS Silicon Avalanche Detectors with Negative Feedback for Time-of-Flight Systems,” by Afanasiev, et. al., published in Nuclear Physics B, Vol. 44, in 1995, and “Localized feedback in silicon-based avalanche photodiodes,” by Khodin, et al., published in Nuclear Instruments and Methods in Physics Research A, Vol. 513, in 2003. In this approach, a so-called “negative feedback” resistor is integrated within a special photodiode structure (referred to as an MRS Photodiode) to passively limit the number of free-carriers generated during an avalanche event. This feedback resistor is, in fact, an integral part of the MRS Photodiode structure. Indeed, MRS is an acronym for Metal-Resistor-Semiconductor, and the operation of an MRS Photodiode relies on the presence of this resistor layer.
However, the MRS structure can only be readily fabricated in the silicon material system. This material system is well-suited to the MRS structure because of the availability of material suitable for forming a resistive layer (namely, silicon carbide (SiC)). Silicon carbide has an appropriate bulk-resistivity and it can be epitaxially-grown on silicon. Unfortunately, the absorption coefficient of silicon drops off sharply as the wavelength of an incident photon approaches 1000 nm. As a result, the silicon-based MRS Photodiode is effectively inoperable at wavelengths greater than 1000 nm.
For operation at wavelengths longer than 1000 nm, photodiodes require materials such as III-V or II-VI semiconductors and/or their compounds. The MRS Photodiode structure is difficult, if not impossible, to fabricate in these material systems, however, since no appropriate resistive layer can be formed as part of the diode structure. This is because these material systems do not exhibit a layer that can be provided with the proper bulk resistivity for a practical resistive layer that can be incorporated into an MRS Photodiode structure.
As a means of quenching avalanche events in canonical SPADs, a discrete resistor that is electrically-connected to the photodiode has been used in the past. Passive quenching has been successfully demonstrated in this manner; however, inevitable large parasitic capacitances associated with the addition of a discrete resistor require very significant discharging and recharging in the avalanche and re-charge cycle of SPAD device operation. The accompanying large current flow results in significant carrier trapping and subsequent afterpulsing effects that, to date, have only been mitigation by employing long “hold-off” times (on the order of 1 microsecond or longer) before re-arming the SPAD. Due to these required hold-off times, the operational rate of these photodetectors has remained limited to approximately 1 MHz or less.
The stifling of trapped charges by lowering the temperature of a SPAD to “freeze” trapped charge carriers has not been successfully demonstrated. In fact, for practical SPAD devices, this approach is likely to increase after-pulsing as temperature is reduced. Further, if carrier freeze-out were successful, it is likely that at least some of the charge carriers associated with the dopant atoms would also be “frozen,” thus rendering the SPAD inoperable.
There exists a need therefore, for a single-photon detector that operates at wavelengths longer than 1000 nm and that can be operated at rates greater than 10 MHz.